In general (but not always), slaves running MySQL 5.1.x can be used with older masters, but not the reverse.Most of us upgrade to a new version by running it on slaves for a long time before putting it on a master. Can we put it on critical slaves? After reading the documentation I am not sure.
An Enterprise database must claim that it supports replication from one release to the next. MySQL seems to support it in practice -- I am able to replicate from 5.0 to 5.1. Am I taking a risk by using 5.1 slaves or does the documentation need an update?